1. Field of the Invention
This invention relates to a valve for electrically controlling the flow rate of a fluid to be supplied from a fluid source to a point of demand for the fluid, and more particularly to a flow control valve which controls the flow rate according to the demand point and which draws in the pressure of the demand point at which a pressure drop below the level of the fluid source occurs under operating conditions, normally maintaining the control valve in an open state as soon as the pressure drop at the demand point is eliminated.
2. Description of the Prior Art
For the purpose of burning combustible gases or decomposing nitrogen oxides in the exhaust gas resulting from operation of an internal combustion engine, it is the practice in the art to control the quantity of air to be mixed with the fuel, generally by means of an electric flow control valve system which supplies air from a by-pass passage to the intake manifold of the engine through a flow control valve according to output signals of a control circuit operating on signals received from an O.sub.2 sensor located in the exhaust manifold. This sort of flow control valve system, for the necessity of supplying a large quantity of air at the time of starting the engine, adds to the control by the output of the O.sub.2 sensor a control of the flow control valve according to the intake manifold vacuum of the engine by drawing the vacuum into a pressure chamber of the valve.
In the conventional electric flow control valve system of the type mentioned above, the control valve is normally held in a closed state by a biasing force of a spring and, when the intake manifold vacuum of the engine is increased such as, for example, of engine starting, opened by the vacuum prevailing in its pressure chamber. When the engine is not in operation, the control valve is closed since no vacuum exists in the intake manifold of the engine. Therefore, there is a possibility of the control valve being frozen in the closed state while the engine is left off in a cold region or climate, failing to open when vacuum is produced in the intake manifold due to starting of the engine.